Recently, in a MOSFET that is used for a memory cell such as a dynamic random access memory (DRAM), a so-called poly-metal structure gate electrode (poly-metal gate) having a refractory metal film such as tungsten stacked on a poly-silicon film is used in order to reduce resistance of a gate electrode (see Japanese Patent Application Laid-Open Nos. 2000-156497 and 2002-141500).
On the other hand, at the time of patterning the gate electrode, a gate dielectric film positioned at a lower part of the gate electrode is also slightly etched. Therefore, a breakdown voltage of the gate dielectric film decreases and a leak current increases in this state. In order to prevent this problem, conventionally, after the gate electrode is patterned, an oxide film is formed on a side surface of the gate electrode. In other words, a so-called light oxidation is carried out.
However, when the gate electrode has a poly-metal structure, a refractory metal scatters to a silicon substrate during the light oxidation, which substantially increases a junction leakage.
As methods for solving this problem, a pamphlet of International Publication No. 98/37583 and Japanese Patent Application Laid-Open Nos. H10-189974 and 2003-68878 disclose techniques of preventing a scattering of a refractory metal film by executing a light oxidation after covering side surfaces of the refractory metal film that constitutes a poly-metal gate with a silicon nitride film.